A. Field of the Invention
The present invention related to a system and method for providing a time varying gain TDR to display abnormalities in a communication and telephone cable or the like by normalizing the levels of the reflected signals corresponding to a predetermined observation range. The communication cable can be any copper pair line that can be used for IDSL, ADSL, HDSL, SDSL, SHDSL, and VDSL communication usage, as well as all other DSL-type technologies (hereinafter, xe2x80x9cxDSLxe2x80x9d indicating all the various DSL technologies and line codings).
B. Description of the Prior Art
Internet access providers, cable communication companies, etc. are constantly working to fill demands for real-time internet access through the installation of fiber-optic and other hi-speed communications lines. However, in many areas of the country, such equipment and the services to support that equipment are either impractical to implement, prohibitively costly, or simply not scheduled to occur in the foreseeable future.
Telephone companies have tried to fill part of the demand by offering Digital Subscriber Line (xDSL) services that use the current infrastructure of copper pair lines to deliver hi-speed access to the Internet. Lines that work fine for standard telephone communication do not always work for different types of DSL operation.
The definition of copper pair lines includes any communication line made of copper or other similar material or composition known in the art. The proper conditions have to exist in order for a copper pair line to handle xDSL communications. The efficiency of a copper pair line for xDSL service is dependent on factors such as the length of the telephone line, the number of bridge taps on the line, material defects or shorts in the line, the wire gauge of the line, damage to the lines, proximity of sources of electromagnetic energy, etc.
Time Domain Reflectometers or TDRs are in common use in testing the abnormality of telephone and coaxial cables, such the TDR described in U.S. Pat. No. 5,461,318 to Borchert et al. (Oct. 24, 1995) which is a method for detecting impedance discontinuities in a two-conductor cable. However, using conventional TDR techniques, this process involves sending service technicians to the ends of a physical line then analyzing the received signal via a local switch. As one can imagine, this entire process is time consuming, labor intensive and costly.
TDRs detect fault anomalies such as opens, shorts, bridged-taps and wet sections. As these lines become longer the loss of the line is higher and it becomes increasingly harder to detect these anomalies. In most cases it takes considerable training and practice to discriminate between these various anomalies. Often even the most experienced telephone technician must drive to other locations, disconnecting sections to resolve their problems.
One key factor in differentiating between a short, a wet section and bridged-tap is the size of the return trace. xe2x80x9cTracexe2x80x9d is a graphical representation of the line voltage verse time. However since the size of this return trace is related to the cable type and distance, or in other words the cable loss (with the return trace decreasing in amplitude as the distance increases), there is no existing method used to definitely resolve these common cable plant problems. Loss with cable plants of mixed cable types, makes detection for these faults even more complex. Detecting multiple faults at different lengths cannot be done on the same trace, as the user must manually set the gain for only a particular range of interest.
Another factor that masks the faults is a phenomenon called back-scatter decay. As cables become longer this returning signal becomes dominant over any detectable fault. Manual gain and offset controls are often required to see faults at longer distances. The user of a traditional TDR have to manually change the gain on the TDR trace in order to see the faults.
Therefore, there currently exists a need for a system and method to test the abnormalities of the copper pair lines that avoids the displaying problems and limitations associated with the current techniques. There also exists a need for a system and method to test the abnormalities of the copper pair lines that can aid in automatically showing abnormalities within a predetermined observation range so as to make the tracing and repair of copper pair lines for xDSL service more efficient.
Time Varying Gain (xe2x80x9cTVGxe2x80x9d), a predetermined gain versus time relationship, has been applied in side scanning sonar systems for mapping the topography of a under water seabed. Acoustic tone bursts (pings) are transmitted through the water column toward a target area and return from the target area are picked up by a receiver transducer and processed for display. Return signals may vary due to unknowns such as temperature, salinity and clarity of the water column. If bottom returns are involved, such as in side looking sonar systems, different bottom types such as mud, sand or rock will return different signals. For instance, U.S. Pat. No. 4,198,702 to Clifford (Apr. 15, 1980) describes a time varying gain amplifier for a side scanning sonar system having a predetermined gain versus time relationship. The gain is substantially proportional to the square of the elapsed time measured from the last sonar trace initiating trigger signal.
U.S. Pat. No. 5,392,257 to Gilmour (Feb. 21, 1995) further provides a sonar receiver with a normalizing processor circuit to modify/normalize the reflected signals to be displayed within the same range by adjusting gain levels. Normalizing processor circuit means is provided and is adapted to receive the signals reflected from unknowns in the water column and various bottom types, and the means is operable to generate an average error signal as a function of time. This error signal is applied to modify the output of the time varying gain circuit.
Adjusting gain levels is common in the art of data processing, and it has been applied to an optical time domain reflectometer (xe2x80x9cOTDRxe2x80x9d) in an optical measurement instrument. U.S. Pat. No. 4,893,006 to Wakai et al. (Jan. 9, 1990) applies such a level adjusting function to an OTDR which works in conjunction with an optical fiber. The optical time domain reflectometer tests a target optical fiber by sending an optical trace to the target optical fiber and detecting Fresnel reflection light and backscattered light returning from the fiber. A level changing means changes the level of the electric signal corresponding to a predetermined location of observation range so as to avoid saturation of the electric signal in the amplifier. As another example, U.S. Pat. No. 5,929,982 to Anderson (Jul. 27, 1999) applies such a level adjusting function (gain control) to an OTDR for optimizing the gain of an active avalanche photo-diode (xe2x80x9cAPDxe2x80x9d). Any system noise is compared to a threshold value for establishing the optimum bias for optimum gain of the APD thereby to increase the dynamic range of the OTDR.
However, the application of TVG to a TDR for telephone lines of the present invention is unique, and the application simplifies the use of TDR in testing for abnormalities in telephone and coaxial cables and enables the display of multiple faults at various cable lengths. In addition, detecting multiple faults at different lengths can be done on the same trace automatically.
It is therefore a general object of the present invention to provide an TDR with Time Varying Gain so as to display the abnormalities of a communication cable or the like in a predetermined observation range.
Another object is to display a reflected trace in predetermined amplitude, regardless of actual amplitude due to cable loss or cable length for a particular given fault (open, short, bridge-tap or wet section). In addition, multiple faults at various lengths can be seen on a single screen.
Another object is to provide a simple method and apparatus for interpreting TDR traces which will then require less user training.
A further object is to eliminate backscatter slope and to highlight faults.
A further object is to easily determine cable type by how well the back-scatter is matched. Further, if a user enters the incorrect wire type, some back-scatter slope will be present, and thus can be corrected. Accordingly, mixed cable types can be identified and compensated for and the need for user intervention in selecting gain manually is eliminated.
A further object is to provide an improved method and apparatus for detecting a physical bridge tap and distinguishing the bridge tap from other types of cable fault.
The present invention involves a copper pair line abnormalities testing apparatus, procedure and protocol. First, parameters such as each copper pair line""s length, wire gauge, impedance, etc. are known, entered in the apparatus, or measured using known measuring techniques. Next, using a time varying gain domain reflectometer (TDR), a known signal or pulse is transmitted through the line; the central office (CO) or customer premise equipment (CPE) can be used as the origin or starting point of the trace signal. The return voltage is measured, wherein impedance mismatches are identified by the characteristics of that return voltage. For example, with a time varying gain TDR device having a display, impedance and wire gauge mismatches can be identified visually by the presence of significant scope or amplitude changes in the graphical representation of the return voltage. The time varying gain TDR""s graphical representations or trace, along with information from other measurements, or manually entered, is normalized to eliminate the effects of the loose cable, namely attenuation and backscatter. The normalization steps will then generate graphical representation indicative of the characteristics of the abnormalities of the copper pair line, wherein the abnormalities may be individually graphically represented in a predetermined observation range. The normalization steps are used to amplify the abnormalities in conjunction with the time interval of the TDR traces and at least one gain coefficient factor.
In accordance with one embodiment of the present invention, a system for displaying abnormalities of a copper pair line comprising at least one time varying gain time domain reflectometer (TDR) having supplying means for supplying at least one pulse of energy at a given pulse width on to a base location of the copper pair line; receiving means for receiving the reflected pulse at the base location; measuring means for measuring the elapsed time from the transmission of the pulse to the receipt of the reflected pulse corresponding to the transmitted pulse; calculating means for calculating the distance from the base location to a abnormality causing the reflected pulse; piecewise gaining and biasing means for piecewise gaining biasing the reflected pulse of various gains to create a first normalized reflected trace which match the reflected pulse of various gains within a predetermined observation range and constitute as one smooth curve; a first time varying gain circuit for amplifying said first normalized reflected trace according to a function of time to create a second normalized reflected trace so as to eliminate an exponential gain decay curve of a no-fault copper pair line with the same predetermined characteristic parameters from said first normalized reflected trace to thereby obtain a second normalized reflected trace showing any amplified abnormalities; and a display for displaying at least one of the reflected trace, the first and second normalized traces corresponding to a predetermined observation range. The abnormalities includes opens, shorts, bridged-taps and wet sections on the wire.
According to a further embodiment of the present invention, a system as described further comprises a second time varying gain circuit for amplifying said second normalized reflected trace according to a function of time to create a third normalized reflected trace so as to amplify the abnormalities thereby to differentiate different types of abnormalities.